This invention relates to a method of detecting deterioration of exhaust gas concentration sensors for internal combustion engines, which are used for the air-fuel ratio feedback control of the engines.
A typical conventional air-fuel ratio feedback control method for internal combustion engines is disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 57-137633. According to this conventional method, a value of concentration of an exhaust gas component (oxygen) is sensed by an exhaust gas concentration sensor arranged in the exhaust system of the engine, and the sensed concentration value is compared with a predetermined reference value. By the use of an air-fuel ratio feedback control correction coefficient based on the result of the comparison, the air-fuel ratio of the mixture supplied to the engine is feedback controlled to a stoichiometric mixture ratio at which can be obtained the best conversion efficiency of a three-way catalyst arranged in the engine exhaust system, thereby improving the exhaust emission characteristics.
An O.sub.2 sensor employed as the exhaust gas concentration sensor in the above system has a sensing element formed of zirconium oxide or the like. Utilizing the fact that the amount of oxygen ion which permeates the interior of zirconium oxide varies depending upon the difference between the partial pressure of oxygen in the atmosphere and the partial pressure of oxygen contained in the exhaust gases, the O.sub.2 sensor senses the exhaust gas oxygen concentration and outputs a voltage which varies as a function of the above-mentioned variation of the permeation amount of oxygen dependent upon the partial pressure difference.
However, it is known that an O.sub.2 sensor of the aforementioned construction has an output characteristic changing with the lapse of time of use. Particularly the sensor output characteristic deteriorates after a vehicle equipped with the sensor has been put through an endurance run, such that the controlled air-fuel ratio becomes richer in comparison with that exhibited at delivery of the vehicle from the factory, even through the feedback control of the air-fuel ratio is performed under the same conditions as at the delivery.
The deteoriorated O.sub.2 sensor output characteristic adversely affects driveability, fuel consumption and exhaust emission characteristics of the engine unless any measures are taken to deal with this aging change in the O.sub.2 sensor output characteristic. To this end, a method of detecting deterioration of an exhaust gas concentration sensor has already been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-189638.
According to this publication, calcuation is made of a ratio of a value obtained by integrating output values from the exhaust gas concentration sensor from the time the output from the sensor is changed or inverted from a leaner side to a richer side with respect to a predetermined reference value to the time the output from same is changed from the richer side to the leaner side with respect to the predetermined reference value, to a value obtained by integrating output values from the sensor from the time the output from the sensor is changed from the richer side to the leaner side with respect to the predetermined reference value to the time the output from same is changed from the leaner side to the richer side with respect to the predetermined reference value. Based on the ratio thus calculated, the predetermined reference value is changed. Detection of deterioration of the sensor due to aging is effected based on the above ratio between the two integral values.
However, the waveform of the output (voltage) from the exhaust gas concentration sensor changes from a steeper one to a gentler one as aging of the sensor proceeds. More specifically, with aging of the sensor, the period of inversion of the output voltage V.sub.02 becomes longer, and at the same time the amplitude of the output voltage becomes reduced. However, according to the above method, in which the deterioration of the O.sub.2 sensor is determined by the above-mentioned ratio between the two integral values, the ratio can remain within a normal range even if the waveform of the output voltage becomes so gentle with aging of the sensor, which makes it impossible to accurately detect of deterioration of the O.sub.2 sensor.